Preclinical evaluation of acoustic radiation force impulse measurements in regions of heterogeneous elasticity

Abstract

Purpose: The purpose of this study was to compare the reliability of ultrasound-based shear wave elastography in regions of homogeneous versus heterogeneous elasticity by using two different probes.

Methods: Using acoustic radiation force impulse (ARFI) elastography, we measured the shear wave velocity (SWV) in different lesions of an elastography phantom with the convex 4C1 probe and the linear 9L4 probe. The region of interest (ROI) was positioned in such a way that it was partly filled by one of the lesions (0%, 25%, 50%, 75%, and 100%) and partly by the background of the phantom (100%, 75%, 50%, 25%, and 0%, respectively).

Results: The success rate was 98.5%. The measured value and the reference value of SWV correlated significantly (r=0.89, P<0.001). Further, a comparison of the two probes revealed that there was no statistical difference in either the mean or the variance values. However, the deviation of SWV from the reference was higher in the case of the 9L4 probe than in the case of the 4C1 probe, both overall and in measurements in which the ROI contained structures of different elasticity (P=0.021 and P=0.002). Taking into account all data, for both probes, we found that there was a greater spread and deviation of the SWV from the reference value when the ROI was positioned in structures having different elastic properties (standard deviation, 0.02±0.01 m/sec vs. 0.04±0.04 m/sec; P=0.010; deviation from the reference value, 0.21±0.12 m/sec vs. 0.38±0.27 m/sec; P=0.050).

Conclusion: Quantitative ARFI elastography was achievable in structures of different elasticity; however, the validity and the reliability of the SWV measurements decreased in comparison to those of the measurements performed in structures of homogeneous elasticity. Therefore, a convex probe is preferred for examining heterogeneous structures.

Keywords: Elasticity imaging techniques; Heterogeneity; Phantoms, imaging; Transducers; Ultrasonography.

Fig. 1.. Principle of acoustic radiation force impulse quantification.

An acoustic push pulse (longitudinal wave) generates minimal deformation of the tissue, inducing shear waves in a user-defined region of interest, which propagate in a perpendicular direction (transversal waves). By applying tracking beams in multiple locations and revealing arrival time, we can quantify the shear wave propagation speed. This correlates closely to the tissue stiffness [22].

Fig. 2.. Experimental set-up.

A, B. Actual set-up (A) and simplified diagram of the set-up (B) are shown. The probe (a) is held by a supporting arm (b). The phantom (c) contains four spherical lesions (diameter: 20 mm, at a depth of 35 mm) of different, defined stiffness values (7.3 kPa, 18.8 kPa, 45.9 kPa, and 61.5 kPa). It is positioned on a digital weighing scale (d).

Fig. 3.. Example of the 50% intralesional positioning of the region of interest (ROI).

The ROI is positioned in such a way that it is partly filled by one of the lesions and partly by the background of the phantom.

Fig. 4.. Measured shear wave velocity (SWV) versus reference value (including all measurements).

The scatter points lie close to the reference line (=non-continuous line), suggesting a high correlation between the measured SWV and the reference value. The interpolants for each probe (=continuous lines) show different patterns: the SWV values from the convex probe end to lie closer to the reference value, whereas the SWV values by the linear probe seem be more frequently underestimated.